docs: def addFilter(self, layer_id, where=None, outFields="*"):
          """ adds a layer definition filter """
          import copy
          f = copy.deepcopy(self._filterTemplate)
          f['layerId'] = layer_id
          f['outFields'] = outFields
          if where is not None:
              f['where'] = where
          if f not in self._filter:
              self._filter.append(f)